Conventionally, an art measuring a thickness of a sheet-shaped object conveyed along a conveyance path with a magnetic sensor is known. For example, arts described in the Patent Literatures 1 and 2 are so.
The art measuring the thickness of the sheet-shaped object conveyed along the conveyance path can be applied to an art detecting whether the sheet-shaped object is plural and overlapped with each other or not on the basis of the measured thickness of the sheet-shaped object.
Each of devices described in the Patent Literatures 1 and 2 has an arm one of whose ends is supported rotatably, a roller rotatably pivoted on the arm and contacting the sheet-shaped object conveyed along the conveyance path, a permanent magnet fixed to the arm, and a magnetic sensor arranged at a position opposite to the permanent magnet (a position at which a magnetic field generated by the permanent magnet can be detected).
According to the devices described in the Patent Literatures 1 and 2, when the thickness of the sheet-shaped object conveyed along the conveyance path is changed, the arm is rotated so as to maintain the state that the roller contacts the sheet-shaped object, and the permanent magnet fixed to the arm is moved relatively to the magnetic sensor. As a result, by changing a distance from the permanent magnet to the magnetic sensor, a magnetic field (magnetic flux density) acting to the magnetic sensor is changed.
The magnetic sensor outputs an electric signal of voltage corresponding to strength of the magnetic field (magnitude of the magnetic flux density) acting to the magnetic sensor.
According to the devices described in the Patent Literatures 1 and 2, as a method improving measurement accuracy (more strictly speaking, measurement accuracy of the thickness of the sheet-shaped object or measurement accuracy of whether the sheet-shaped object is plural and overlapped with each other or not), following methods (1) to (3) can be considered generally.
(1) By increasing “moving distance of the permanent magnet per unit rotation angle of the arm”, “change amount of the magnetic field acting to the magnetic sensor” per the moving distance of the permanent is increased.
(2) By selecting the permanent magnet which can generate the larger magnetic field, “the change amount of the magnetic field acting to the magnetic sensor” per the moving distance of the permanent is increased.
(3) By selecting the magnetic sensor with higher sensitivity, detection of slight change of the magnetic field is enabled.
However, in the case of (1), a full length of the arm becomes long, whereby the whole device is enlarged.
In the case of (2), the permanent magnet which can generate the stronger magnetic field (magnetic flux density) is generally more expensive than the magnet which spreads widely, whereby production cost of the device is increased.
In the case of (3), the magnetic sensor with the high sensitivity is generally expensive, whereby the production cost of the device is increased.
The strength of the magnetic field (the magnitude of the magnetic flux density) generated by one (single) magnet is decreased suddenly as the distance from the permanent magnet is increased, whereby the change of the magnetic flux density per the moving distance can be increased only in the case that the permanent magnet is arranged very near the magnetic sensor.
On the other hand, an art that the thickness of the sheet-shaped object conveyed along the conveyance path is measured in a non-contact state with an ultrasonic wave sensor is known. However, such an ultrasonic wave sensor is expensive and causes increase of the production cost.